The effects of pressure on the yields of polycyclic aromatic hydrocarbons produced during the supercritical pyrolysis of toluene

To understand better the reactions leading to polycyclic aromatic hydrocarbons (PAH) - and ultimately carbonaceous solids - within the context of supercritical fuel pyrolysis, we have pyrolyzed the model fuel toluene (critical temperature, 319 °C; critical pressure, 41 atm) in an isothermal silica-lined stainless steel coil reactor at 535 °C, 140 s, and pressures of 20-100 atm. Analysis of the reaction products by gas chromatography and high-pressure liquid chromatography reveals that the yields of benzene and 27 individual PAH increase exponentially with pressure. For 26 of these 28 products, the experimentally measured yield/ pressure data conform well to a first-order global kinetics model - permitting determination of the pre-exponential factor A and the activation volume ΔV^≠, which appear in the pressure-dependent expression for the kinetic rate constant: κ = Aexp[(-ΔV^≠/RT)p]. For most of the PAH, derived values of ΔV^≠ lie between -2.5 and -4 L/mol, signifying the doubling of PAH formation rates by pressure increases of only 18.4 or 11.5 atm, respectively. Some of the larger PAH, such as the 8-ring benzo[a]coronene and the 9-ring naphtho[8,1,2-abc]coronene, exhibit negative activation volumes of even greater magnitude - a result of particular relevance to the formation of carbonaceous solids, as large PAH are thought to be precursors to these solids. The PAH yield data also reveal product yield ratios, within certain PAH isomer families, that are peculiar to the high-pressure supercritical pyrolysis environment - suggesting that mechanisms for PAH formation differ significantly from those in atmospheric-pressure gas-phase pyrolysis environments.